Numerical Flow Visualization Research Articles

유동가시화를 통한 다중 수중익 덕트 내 유속조절에 대한 연구

In this work, we investigate the flow velocity controllability of a diffuser-type multiple hydrofoil duct by experimental and numerical flow visualization approaches. The flow velocity controllability is analyzed by changing the angle of the hydrofoil near the outlet, which is the diffuser, while the incoming flow velocity is 0.6 m/s in the experiment. When the diffuser angle is changed from 0 to 7.5 degree, the maximum velocity inside the duct is varied from 1.35 m/s to 1.52 m/s. Also, it is shown from the numerical analysis that the maximum velocity is varied from 1.09 m/s to 1.17 m/s in the same condition. Thus, the aspect of the acceleration in the duct due to the increase of the diffuser angle is similar between the both approaches. Therefore, the multiple hydrofoil duct can be used to control the flow speed inside the duct for continuously extracting power close to a rated capacity.

Three-dimensional direct numerical simulation of wake transitions of a circular cylinder

This paper presents three-dimensional (3D) direct numerical simulations (DNS) of flow past a circular cylinder over a range of Reynolds number ($Re$) up to 300. The gradual wake transition process from mode A* (i.e. mode A with large-scale vortex dislocations) to mode B is well captured over a range of $Re$ from 230 to 260. The mode swapping process is investigated in detail with the aid of numerical flow visualization. It is found that the mode B structures in the transition process are developed based on the streamwise vortices of mode A or A* which destabilize the braid shear layer region. For each case within the transition range, the transient mode swapping process consists of dislocation and non-dislocation cycles. With the increase of $Re$, it becomes more difficult to trigger dislocations from the pure mode A structure and form a dislocation cycle, and each dislocation stage becomes shorter in duration, resulting in a continuous decrease in the probability of occurrence of mode A* and a continuous increase in the probability of occurrence of mode B. The occurrence of mode A* results in a relatively strong flow three-dimensionality. A critical condition is confirmed at approximately $Re=265{-}270$, where the weakest flow three-dimensionality is observed, marking a transition from the disappearance of mode A* to the emergence of increasingly disordered mode B structures.

Development of a bi-directional electrohydrodynamic pump: Parametric study with numerical simulation and flow visualization

We propose a bi-directional electrohydrodynamic pump developed for transporting dielectric liquid, where the electrodes are symmetrically configured but the applied voltage is non-symmetric. The underlying principle for liquid transport comes from the so-called Onsager effect, which states that the ion concentration is increased as the electric field is increased. Multi-physics software is used to perform numerical simulation for the fluid flow, the electric potential, and the transport of ion concentrations for two kinds of electrode patterns. A flow-visualization experiment is also conducted to verify the physical models and numerical methods employed. It is found that significant reduction of the ion recombination constant is required to get matching of the experimental and simulation results. We demonstrate through a parametric study that there is an optimum distance between two large grounded electrodes for producing a maximum pumping velocity at the diameter of two small electrodes fixed at 0.3 mm. The effect of the size of large grounded electrodes on the pumping performance is also studied in terms of streamlines, electric field, and charge distribution. A general account is also given of the basic ideas of electrode arrangement for the enhancement of pumping.

Numerical and experimental study of shock waves emanating from an open-ended rectangular tube

We examine the dynamics of a high-speed shock-induced flow near the open end of a shock tube using the particle image velocimetry (PIV) and the background oriented schlieren (BOS) methods along with two- and three-dimensional numerical simulations. In experiments, planar shock waves (\(M=1.3\)–1.6) are discharged from a rectangular (\(24\,\hbox {mm} \times 48\,\hbox {mm}\)) low-pressure section of a shock tube open to the atmosphere. Due to the rectangular exit geometry, the resulting flow is highly three-dimensional and, thus, more complicated, compared to well-studied circular/axisymmetric geometries. The study focuses on the spatio-temporal flow structure up to 1 ms after the shock wave diffraction. PIV and BOS visualization techniques share the same post-processing principle, and the iterative multi-step cross-correlation algorithm applied in the PIV software is adapted here for the calculation of background pattern displacement on the BOS images. Particular attention is given to the resolution of flow regions where sharp gradients are present, such as a diffracted shock front or embedded shocks. Computational fluid dynamic simulations of the problem are also conducted to validate the experimental results and methods and to gain more insight into the three-dimensional flow dynamics. PIV and BOS images are found to be consistent with the corresponding numerical flow visualizations.

Aerodynamic performance of a circulating airfoil section for Magnus systems via numerical simulation and flow visualization

The lift to drag ratio is the most determining factor in power efficiency of wind conversion systems such as wind turbines. The lift to drag ratio can be enhanced considerably using circulating airfoil shapes. In this research, the NACA0021 airfoil is modified and manufactured to form a treadmill like circulating shape. Aerodynamic performance of the treadmill shape is computationally and experimentally investigated. In the experiment, the smoke flow visualization is conducted in a small wind tunnel to study flow features such as separation and stagnation points. Computational method is based on the finite volume discretization of RANS (Reynolds Averaged Navier–Stokes) equations and the shear stress transport (k–ω SST (shear-stress transport)) turbulence modeling. Magnus effects are investigated under various speed ratios (speed of circulating surface to the wind speed) and several angles of attack. The numerical and flow visualization results reveal some main changes to flow features including full removal of separation zone above moving surfaces. At the speed ratio of 3, zero angle of attack and flow Reynolds number of 94,000, an impressive lift to drag ratio of 130 is obtained for the circulating airfoil whilst the maximum attainable value is merely 45 for the original NACA0021 airfoil.

Analysis of self-excited forces for a box-girder bridge deck through unsteady RANS simulations

This paper presents the results of two-dimensional numerical simulations of the flow field around a trapezoidal box-girder bridge section with later cantilevers, experiencing small-amplitude heaving or pitching harmonic oscillations. Unsteady Reynolds-averaged Navier–Stokes equations are solved in conjunction with an eddy-viscosity and an explicit algebraic Reynolds stress model. Flutter derivatives are determined and compared with wind tunnel results, showing fairly good agreement. The degree of sharpness of the deck lower edges is found to play a key role in the aeroelastic behavior of the profile. In particular, the bridge section fully behaves as a bluff body and is prone to low-reduced-wind-speed torsional galloping in the case of perfectly sharp edges. By contrast, the presence of a small radius of curvature in the section lower corners changes the nature of the instability to coupled flutter and significantly postpones the stability threshold, in line with a quasi-streamlined body behavior. Moreover, a wide sensitivity study is carried out, investigating the influence on the self-excited forces of the amplitude of oscillation, mean angle of attack and Reynolds number. In particular, the numerical simulations for the geometry with smooth lower edges highlight the regime transition occurring when the Reynolds number is varied, with significant effects on the flutter derivatives. Finally, the numerical flow visualizations provide a physical explanation of some phenomena observed in the wind tunnel experiments.

A New Design for Enhancing the Performance of Hidden Ceiling Fan

The new axial-flow fans are designed by following a systematic procedure to construct the 3D multi-airfoil impeller, which is constructed by NACA four-digital series of 2D profiles. Also a new housing is proposed for enhancing the performance. The numerical flow visualization is observed carefully for modifying the design parameters when the undesirable flow pattern appeared. After that a CNC-fabricated prototype consisting of new housing and axial-flow fan is served for performance comparison. The results show that data from experiment and simulation have the same trend and in good agreement. Furthermore, the airflow rate for mockup is largely improved by 64.57% after imposing the new axial-flow fan and the new housing. Moreover, for design of single axial-flow fan, the maximum static pressure and maximum airflow rate are increased by a significant 33.7% and 55.48%, both much higher than the commercial products, respectively. In conclusion, the enhancement of hidden ceiling fan has been accomplished successfully via the systematic scheme proposed in this effort.

Numerical analysis of hypersonic flows around blunt-nosed models and a space vehicle

This work addresses the problem of the aerothermodynamics of hypersonic nonequilibrium flows over blunt nosed models and space vehicles with rarefaction effects. First, the in-house Navier–Stokes solver, UNIC-UNS code, with the slip boundary condition and finite-rate chemistry is used to simulate the hypersonic flows over a blunt nosed model and the simplified European eXPErimental Re-entry Test-bed (EXPERT) model V4.4. Next, hypersonic flows over the whole EXPERT 3D model, which correspond to the expected descent trajectory with allowance for rarefaction and thermochemical nonequilibrium are simulated. By comparing with the Direct Simulation Monte Carlo (DSMC) method, it is observed that the UNIC-UNS code is reliable in simulating hypersonic flows with rarefaction and thermochemical non-equilibrium effects. A detailed analysis of the aerothermodynamics for EXPERT for a wide range of flow regimes is also provided by utilizing the numerical flow visualization. The present numerical simulations provide some important data for EXPERT, which cannot be easily derived by experiments. This study aims to work as a precursor for future studies and to provide to the scientific community with quality data that can be used to improve tools for the design of hypersonic vehicles.

Numerical investigation of flow and scour around a vertical circular cylinder.

Flow and scour around a vertical cylinder exposed to current are investigated by using a three-dimensional numerical model based on incompressible Reynolds-averaged Navier-Stokes equations. The model incorporates (i) k-ω turbulence closure, (ii) vortex-shedding processes, (iii) sediment transport (both bed and suspended load), as well as (iv) bed morphology. The influence of vortex shedding and suspended load on the scour are specifically investigated. For the selected geometry and flow conditions, it is found that the equilibrium scour depth is decreased by 50% when the suspended sediment transport is not accounted for. Alternatively, the effects of vortex shedding are found to be limited to the very early stage of the scour process. Flow features such as the horseshoe vortex, as well as lee-wake vortices, including their vertical frequency variation, are discussed. Large-scale counter-rotating streamwise phase-averaged vortices in the lee wake are likewise demonstrated via numerical flow visualization. These features are linked to scour around a vertical pile in a steady current.

The Effect of Break Edge Configuration on the Aerodynamics of Anti-Ice Jet Flow

One of the components of a turboprop gas turbine engine is the Front Bearing Structure (FBS) which leads air into the compressor. FBS directly encounters with ambient air, as a consequence ice accretion may occur on its static vanes. There are several aerodynamic parameters which should be considered in the design of anti-icing system of FBS, such as diameter, position, exit angle of discharge holes, etc. This research focuses on the effects of break edge configuration over anti-ice jet flow. Break edge operation is a process which is applied to the hole in order to avoid sharp edges which cause high stress concentration. Numerical analyses and flow visualization test have been conducted. Four different break edge configurations were used for this investigation; without break edge, 0.35xD, 74xD, 0.87xD. Three mainstream flow conditions at the inlet of the channel are defined; 10m/s, 20 m/s and 40 m/s. Shear stresses are extracted from numerical analyses near the trailing edge of pressure surface where ice may occur under icing conditions. A specific flow visualization method was used for the experimental study. Vane surface near the trailing edge was dyed and thinner was injected into anti-ice jet flow in order to remove dye from the vane surface. Hence, film effect on the surface could be computed for each testing condition. Thickness of the dye removal area of each case was examined. The results show noticeable effects of break edge operation on jet flow, and the air film effectiveness decreases when mainstream inlet velocity decreases.

The stability of a thin water layer over a rotating disk revisited

The flow driven by a rotating disk of a thin fluid layer in a fixed cylindrical casing is studied by direct numerical simulation and experimental flow visualizations. The characteristics of the flow are first briefly discussed but the focus of this work is to understand the transition to the primary instability. The primary bifurcation is 3D and appears as spectacular sharp-cornered polygonal patterns located along the shroud. The stability diagram is established experimentally in a (Re, G plane, where G is the aspect ratio of the cavity and Re the rotational Reynolds number and confirmed numerically. The number of vortices scales well with the Ekman number based on the water depth, which confirms the existence of a Stewartson layer along the external cylinder. The critical mixed Reynolds number is found to be constant as in other rotating flows involving a shear-layer instability. Hysteresis cycles are observed highlighting the importance of the spin-up and spin-down processes. In some particular cases, a crossflow instability appears under the form of high azimuthal wave number spiral patterns, similar to those observed in a rotor-stator cavity with throughflow and coexists with the polygons. The DNS calculations confirm the experimental results under the flat free surface hypothesis.

Macro- and microscale variables regulate stent haemodynamics, fibrin deposition and thrombomodulin expression

Drug eluting stents are associated with late stent thrombosis (LST), delayed healing and prolonged exposure of stent struts to blood flow. Using macroscale disturbed and undisturbed fluid flow waveforms, we numerically and experimentally determined the effects of microscale model strut geometries upon the generation of prothrombotic conditions that are mediated by flow perturbations. Rectangular cross-sectional stent strut geometries of varying heights and corresponding streamlined versions were studied in the presence of disturbed and undisturbed bulk fluid flow. Numerical simulations and particle flow visualization experiments demonstrated that the interaction of bulk fluid flow and stent struts regulated the generation, size and dynamics of the peristrut flow recirculation zones. In the absence of endothelial cells, deposition of thrombin-generated fibrin occurred primarily in the recirculation zones. When endothelium was present, peristrut expression of anticoagulant thrombomodulin (TM) was dependent on strut height and geometry. Thinner and streamlined strut geometries reduced peristrut flow recirculation zones decreasing prothrombotic fibrin deposition and increasing endothelial anticoagulant TM expression. The studies define physical and functional consequences of macro- and microscale variables that relate to thrombogenicity associated with the most current stent designs, and particularly to LST.

Bejan's heatlines and numerical visualization of convective heat flow in differentially heated enclosures with concave/convex side walls

Numerical simulation for natural convection flow in fluid filled enclosures with curved side walls is carried out for various fluids with several Prandtl numbers (Pr = 0.015, 0.7 and 1000) in the range of Rayleigh numbers (Ra = 103–106) for various cases based on convexity/concavity of the curved side walls using the Galerkin finite element method. Results show that patterns of streamlines and heatlines are largely influenced by wall curvature in concave cases. At low Ra, the enclosure with highest wall concavity offers largest heat transfer rate. On the other hand, at high Ra, heatline cells are segregated and thus heat transfer rate was observed to be least for highest concavity case. In convex cases, no significant variations in heat and flow distributions are observed with increase in convexity of side walls. At high Ra and Pr, heat transfer rate is observed to be enhanced greatly with increase in wall convexity. Results indicate that enhanced thermal mixing is observed in convex cases compared to concave cases. Comparative study of average Nusselt number of a standard square enclosure with concave and convex cases is also carried out. In conduction dominant regime (low Ra), concave cases exhibit higher heat transfer rates compared to square enclosure. At high Ra, low Pr, concave cases with P1P1′=0.4 is advantageous based on flow separation and enhanced local heat transfer rates.

Performance improvements of a subsonic axial-flow compressor by means of a non-axisymmetric stator hub end-wall

This paper deals with the application of a non-axisymmetric hub end-wall on the stator of a single stage high subsonic axial-flow compressor. In order to obtain a state-of-the-art stator non-axisymmetric hub end-wall configuration fulfilling the requirements for higher efficiency and total pressure ratio, an automated multi-objective optimizer was used, in conjunction with 3D-RANS-flow simulations. For the purpose of quantifying the effect of the optimal stator non axis-symmetric hub contouring on the compressor performance and its effects on the subsonic axial-flow compressor stator end-wall flow field structure, the coupled flow of the compressor stage with the baseline, axisymmetric and the non-axisymmetric stator hub end-wall was simulated with a state-of-theart multi-block flow 3D CFD solver. Based on the CFD simulations, the optimal compressor hub end-wall configuration is expected to increase the peak efficiency by approximately 2.04 points and a slight increase of the total pressure ratio. Detailed analyses of the numerical flow visualization at the hub have uncovered the different hub flow topologies between the cases with axisymmetric and non-axisymmetric hub end-walls. It was found that that the primary performance enhancement afforded by the non-axisymmetric hub end-wall is a result of the end-wall flow structure modification. Compared to the smooth wall case, the non-axisymmetric hub end-wall can reduce the formation and development of in-passage secondary flow by aerodynamic loading redistribution.

Numerical Simulation of Airfoil Aerodynamic Penalties and Mechanisms in Heavy Rain

Numerical simulations that are conducted on a transport-type airfoil, NACA 64-210, at a Reynolds number of2.6×106and LWC of 25 g/m3explore the aerodynamic penalties and mechanisms that affect airfoil performance in heavy rain conditions. Our simulation results agree well with the experimental data and show significant aerodynamic penalties for the airfoil in heavy rain. The maximum percentage decrease inCLis reached by 13.2% and the maximum percentage increase inCDby 47.6%. Performance degradation in heavy rain at low angles of attack is emulated by an originally creative boundary-layer-tripped technique near the leading edge. Numerical flow visualization technique is used to show premature boundary-layer separation at high angles of attack and the particulate trajectories at various angles of attack. A mathematic model is established to qualitatively study the water film effect on the airfoil geometric changes. All above efforts indicate that two primary mechanisms are accountable for the airfoil aerodynamic penalties. One is to cause premature boundary-layer transition at low AOA and separation at high AOA. The other occurs at times scales consistent with the water film layer, which is thought to alter the airfoil geometry and increase the mass effectively.

Wave run-up on columns of deepwater platforms

Structural design of novel deepwater platforms is an outcome of innovation and optimization. Structures such as the extended Tension Leg Platform have columns of rectangular cross section, while platforms like spars are circular. An important operational requirement of these platforms is to have restricted heave motion to support vertical rigid risers. An unfortunate consequence of this requirement is the so-called breakwater effect, which results in significant wave run-up on the structure. In high sea states, considerable water could run up along the columns hitting the underside of horizontal deck. Estimating wave run-up on offshore structures is an essential part of deciding the minimum air-gap requirements of such structures. This article examines the run-up on circular and square cylinders using experimental and numerical methods. Experiments were performed in a wave tank on scaled models of prototype circular and square columns. Numerical simulations were conducted using the commercial software FLUENT. It was found that that the results for wave run-up from simulations and experiments were in good agreement and were consistently greater than linear diffraction theory. Numerical comparison between circular and square cylinders showed that the run-up was higher for square geometry for all conditions simulated. This increase was higher in steeper waves due to various nonlinearities in the flow around the structure. An attempt has been made to understand the various nonlinearities in the run-up profile using numerical flow visualization.

Numerical investigation of flow past single and tandem cylindrical bodies in the vicinity of a plane wall

A numerical study has been conducted to investigate the behavior of the vortical wake created by square and circular cylinders placed in a boundary-layer flow formed over a plane wall. The calculations are performed by solving the unsteady two dimensional (2-D) Navier–Stokes equations with a finite volume method at low Reynolds numbers, Re=100 and 200. Apart from the Reynolds number the flow field characteristics also depend on distance of the cylinder from the plane wall and longitudinal gap between two cylinders, L. To study the effect of gap (G) between the cylinder and the wall, calculations are performed at various gap ratios (g⁎). Gap ratio is defined as the ratio of the gap (G) and the characteristic cross-stream dimension of the cylinder. In the case of square cylinder the characteristic dimension is the length of a side of the cylinder (H) and in the case of a circular cylinder the dimension is the cylinder diameter (D). The vorticity in the boundary layer formed over the plane wall interacts intensely with the vorticity associated with the shear layer emanating from the separation points on the cylinder surface and produces a complex flow field in the wall gap and the cylinder wake. An attempt has been made to analyze these complex flow features through numerical flow visualization tools like vorticity contours and streamlines.

Characterization of the flow past a truncated square cylinder in a duct under a spanwise magnetic field

AbstractWe study the flow of an electrically conducting fluid past a truncated square cylinder in a rectangular duct under the influence of an externally applied homogeneous magnetic field oriented along the cylinder axis. Our aim is to bridge the gap between the non-magnetic regime, where we previously found a complex set of three-dimensional recirculations behind the cylinder (Dousset & Pothérat,J. Fluid Mech., vol. 653, 2010, pp. 519–536) and the asymptotic regime of dominating Lorentz force analysed by Hunt & Ludford (J. Fluid. Mech., vol. 33, 1968, pp. 693–714). The latter regime is characterized by a remarkable structure known asHunt’s wakein the magnetohydrodynamics community, where the flow is deflected on either side of a stagnant zone, right above the truncated cylinder as if the latter would span the full height of the duct. In steady flows dominated by the Lorentz force, with negligible inertia, we provide the first numerical flow visualization of Hunt’s wake. In regimes of finite inertia, a thorough topological analysis of the steady flow regimes reveals how the Lorentz force gradually reorganizes the flow structures in the hydrodynamic wake of the cylinder as the Hartmann number$\mathit{Ha}$(which gives a non-dimensional measure of the magnetic field) is increased. The nature of the vortex shedding follows from this rearrangement of the steady structures by the magnetic field. As$\mathit{Ha}$is increased, we observe that the vortex street changes from a strongly symmetric one to the alternate procession of counter-rotating vortices typical of the non-truncated cylinder wakes.

Investigation of Size Effects to the Mixing Performance on the X-shaped Micro-Channels

Due to the developing of micro-electro-mechanical-system, MEMS, the fabrication of the microminiaturization devices becomes obviously important. The advances in the basic understanding of fluid physics have opened an era of application of fluid dynamics systems using microchannels. The purpose of this study is to research the flow transport phenomenon by employing different kinds of micro-channel sizing in X-shaped micro-channels. As the working fluid, water is injected to microchannel at different mass flow rate. Over a wide range of flow condition, 1.06 < Re < 514, in X-shaped micro-channels, the mixture performances of numerical simulation, flow visualization, and temperature distribution remain the same. At the same mass flow rate as the Reynolds number below 112.53, the biggest channel size had the slowest flow velocity and got the best mixing performance; as the Reynolds number above 112.53, the smaller the channel sizing, the lower the pressure drops and the faster velocity becomes. The transition form early from laminar flow, the unsteady flow is an advantage for mixing in the limited mixing area, therefore 0.7 mm got the best mixing performance. It is clear that the size of the channel plays an important role in the X-shaped micro-channels.

Numerical simulation and flow visualization using soap film of the self-organized vortex structure in the wake of an array of cylinders

With the aid of computational fluid dynamics (CFD) and simple flow visualization technique using flowing soap-film, we present here the wake structures behind an array of cylinders for Reynolds numbers corresponding to both laminar and turbulent flow regimes. The image results illustrate interesting vortex interactions past these equally spaced cylinders; for low Reynolds number flow, well-organized wake pattern persists and manifests unsteadily to different symmetry states. An increase of free stream flow velocity causes the wake transition, resulting in the formation of asymmetric flow wake with chaotic mixing at the far wake. Observations from both the numerical simulations and soap-film are in good agreement at least qualitatively.